The structure of peripheral nervous system myelin differs from that of the central nervous system in: (a) the satellite cells that produce the myelin sheaths; (b) the radial structural periods of the concentric cylindrical lamellar arrays; (c) the degree of symmetry of the individual membranes of the membrane-pair units; (d) the relative amounts of three kinds of protein and of various lipids present; and (e) the relative affinities for insertion of water between the external appositions of the membrane-pairs to cause swelling. In general, central myelin is more compact, its membranes are more nearly symmetrical, and swelling is not as readily achieved as with peripheral myelin, which may be normally slightly swollen and readily accepts aqueous layers between the membrane-pairs. This project is assembling and applying recording instrumentation for x-ray diffraction studies of the stability of myelin as a function of time and environment; the structural variations which accompany known compositional differences occurring over mature mammalian nervous systems and during early post-natal development; the species variations between myelins of vertebrate animals, including comparatively rare cases of myelination in invertebrates; and the relationship of lipid and protein distribution in myelin membrane-pair units to these biological observations.